1. Field of the Invention
The present invention relates to a semiconductor memory device, a method of writing data therein, and a method of reading data therefrom.
2. Description of the Related Art
JP 2006-85753 A and JP 2006-338773 A each describe a conventional semiconductor memory device, for example. The semiconductor memory devices described in those documents each have a plurality of memory chips. Each memory chip has storing means where information for identifying the memory chip (chip ID) is stored. The storing means includes a plurality of fuses. The chip ID is adapted to be written by disconnecting/connecting the fuses. Specifically, whether the respective fuses have data of 0 or 1 is determined by whether the respective fuses are completely disconnected or not.
It is to be noted that, in addition to JP 2006-85753 A and JP2006-338773A, JP2005-39220A is also given as the related art of the present invention.
The present inventor has recognized as follows. Even if the fuses are disconnected, under the influence of electromigration or the like, the disconnected portions may be connected again. In that case, the data of the fuses (0 or 1) are inverted. Therefore, the semiconductor memory device described above is susceptible to improvement with regard to reliability.